Polyester fiber which is represented by polyethylene terephthalate fiber is used widely for apparel and industrial materials. In recent years, requirements toward higher performance of the industrial fibers, particularly the tire-reinforcing fibers, has been heightening and fiber with good dimensional stability against heat is being desired.
Attempts to make polyester fiber of improved dimensional stability have been made in various ways. For example, in JP No. 528 - 1988 and JP No. 529 - 1988, the method in which the undrawn yarn of high orientation is made by increasing the spinning speed and then drawing in continuation in making the polyester fiber was proposed. However, the fiber which is obtained by this method has large shrinkage and the dimensional stability is not adequate.
Also, in Kokai JP No. 259620 - 1985, the method in which melt spinning is done at high speed and the yarn is taken up and then this is drawn in multiple stages to make polyester fibers of high initial Young's modulus and low shrinkage was proposed. The yarn which is obtained by this method, however, has high birefringence and has high amorphous orientation perhaps because of improper distribution of draw ratios and so the shrinkage is high and dimensional stability is inferior.
Also in Kokai JP No. 165547 - 1988, the polyester tire cord of high modulus and the method of making are described and the method in which, in making the polyester fiber, melt spinning is done at a spinning speed of over 5000 m/min and then hot drawing to a ratio of 1.2-1.8 is done is disclosed; but, this is the so called split process method and there is a problem in the cost.
Also, in Kokai JP No. 159518, an invention related to thermally stable polyester fiber is disclosed; however, the strength is low and it is not suitable as industrial fiber.
This invention is intended to make it possible to make the polyester fiber which has good dimensional stability against heat and is suitable as an industrial fiber with a high productivity.